1. Field
The disclosed concept pertains generally to direct current to direct current (DC/DC) converters and, more particularly, to multi-stage DC/DC converters. The disclosed concept also pertains to methods of controlling DC/DC converters.
2. Background Information
A DC/DC converter is configured to receive an input direct current (DC) voltage and convert it into one or more output DC voltages. In many applications, such as electric vehicle chargers, a DC/DC converter must be able to provide a relatively wide output voltage range. For example, the electric vehicle charging voltage range required by the CHAdeMo specification is 50-500 VDC. In other words, the maximum voltage in the range is ten times the minimum voltage in the range.
One type of DC/DC converter is an LLC resonant converter. An LLC resonant converter is a type of resonant converter whose resonant frequency is determined by two inductive components and one capacitive component. LLC resonant converters provide high efficiency, low levels of EMI emissions, high power density, and low cost. However, in prior LLC resonant converter designs, increasing the output voltage range detrimentally affects the efficiency of the LLC resonant converter by causing a larger shunt current in the primary side, thus increasing conduction loss.
Some prior DC/DC converter designs have used a buck stage in conjunction with an LLC resonant converter stage to obtain a wider output range. In one prior configuration, a buck stage is added after the LLC resonant converter stage, thus allowing a wider output voltage range.
In such DC/DC converter designs, the output voltage of the LLC resonant converter stage is controlled by changing its switching frequency and the output voltage of the buck stage is controlled by changing its duty cycle. Two methods have been used to control the output voltage of the DC/DC converter in these designs. For example, if the required output voltage is 50-500 VDC and the maximum current is 30 A, a first method fixes the output voltage of the LLC resonant converter stage to 500 VDC regardless of the load and regulates the output voltage with the buck stage. With this method, the LLC resonant converter stage can be optimized to have a high efficiency. However, the buck stage must have a relatively wide output range of 50-500 VDC and must be able to handle power up to 15 kW. Due to its hard switching, the loss in the buck stage is large. Additionally, the buck stage must be designed for a power of 15 kW, thus increasing its cost.
In a second method, the output voltage of the LLC resonant converter stage has a regulation range of 310-500 VDC at any load from OA to 30 A. If the required output voltage is above 310 VDC, the duty of the buck stage is set to 100% and the output voltage is regulated by the LLC resonant converter stage. If the required output voltage is below 310 VDC, the output voltage of the LLC resonant converter stage is set to 310 VDC or some other value at which the LLC resonant converter stage has a relatively high efficiency and the output voltage is regulated by controlling the duty cycle of the buck stage. In this second method, the efficiency of the LLC resonant converter stage is lower than the efficiency of the LLC resonant converter stage of the first method. However, the buck stage regulates a narrower range of voltages and has a lower power rating. Although the second method is more efficient than the first method, there is room for further improvement in DC/DC converters.